Autoxidizable alkyds are long-established binders for film-forming coating compositions acknowledged for their esthetical properties, a low surface tension, which enables the wetting of and adhesion on a wide variety of substrates and facilitates pigment wetting, can be applied by various techniques, and are cost-effective.
The environmental burden and health problems caused by solvent emission imposes the development of coating formulations containing little or no volatile organic compound (VOC) content. However, while the move to reduced organic solvent-based compositions brings health and safety benefits, these lower VOC content alkyd coating compositions must still meet or exceed the performance standards expected from solvent-based compositions. Such efforts by the coatings industry has resulted in the development of a number of high-solids resin coating formulations which contains significantly reduced amounts of solvents, and, in some instances, little or no solvent.
Alkyd resins generally require transition metal soaps which act as driers, particularly a cobalt soap, in combination with a zirconium soap, to catalyze the polymerization of drying oils used in such resin compositions. However, many of the drier catalysts which have been useful as driers for conventional solvent based coating formulations are observed to be inadequate when used in high-solids coating formulations. High solid alkyd resins have relatively long “dry” and/or cure times, particularly at ambient temperatures or cold temperatures.
In addition, while the metal soaps perform quite effectively when they are first compounded in such coating compositions, these compositions in a formulation having high solids content, often exhibit a reduction in drying rate on aging, compared to conventional solids formulations. For example, when an alkyd paint is highly pigmented, a phenomenon often occurs which has been called “loss of dry”—that is, the alkyd paint does not dry as rapidly after prolonged storage as when first prepared. For example, a paint which may dry in 4 hours when first prepared, may require 6 to 8 hours or longer for drying after 30 days of storage.
It has been suggested that the reduction in the drying rate results from an adsorption of the metal soaps on the surface of pigment particles and extenders used in the coating composition, or possibly by a reaction between the metal soaps and other coating constituents that produce insoluble, and inactive metal compounds. Whatever the reason, the reduction in the drying rate of such coating compositions is generally considered a disadvantage.
Certain “loss of dry” inhibitors have been available in the prior art. U.S. Pat. No. 3,901,837, for example, describes a solution of a mixture of cobalt, calcium and zinc, or zirconium, oil-soluble metal salts (soaps) as a loss-of-dry inhibitor. U.S. Pat. No. 3,972,825 describes loss-of-dry inhibitors comprising metal salt solutions that contain cobalt, calcium, and zinc or zirconium. WO98/53018 describes a composition comprising a reaction product of an oil soluble cobalt soap and an oil soluble over-based calcium soap which serves as a “loss of dry” inhibitor, The loss-of-dry inhibitors function to counteract the loss of activity observed when using the conventional metal soaps, but none has proven to be entirely satisfactory.
There remains a need in the market to further improve the surface dry time of low VOC alkyd based coating compositions to a very low level, in particular at room or low temperatures, and to minimize or prevent the loss of drying rate on aging of alkyd based-coating compositions. It is accordingly one of the objects of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.